1. Field of the Invention
This invention is directed to gas operated valve actuators and to valve systems with such operators.
2. Description of Related Art
Certain prior art gas-operated valve actuators use O-ring seals in a variety of locations. One example of such a system is shown in U.S. Pat. No. 2,743,897. A problem is encountered when these O-rings are sheared, e.g. by a moving valve spool forcing an O-ring against a projection thread, slot, keyway, spline, corner, edge, port or angled surface, particularly when an O-ring has expanded due to heat or by gas permeation. Sheared O-rings result in a variety of unwanted results, including line pressure gas leaking to a vent or exhaust which then makes it impossible for the actuator to operate the main valve. Leaking gas also pollutes the environment.
A variety of attempts have been made to overcome the O-ring shear problem, including: adding a chamfer to edges contacting or contacted by an O-ring; using O-rings made of relatively harder material; and use of a "floating groove" so that sealing is accomplished by peripheral squeezing applied to surface of the O-rings outside circumference as it enters the bore of the spool valve body and gas pressure moves the O-ring into facial contact with the wall of the groove. Even with the use of a floating groove, O-ring dimensions change, gas permeates the O-rings, gas trapped in an O-ring expands rapidly changing O-ring size, and O-rings swell due to a reaction to ingredients in the gas (e.g. additives, corrosion inhibitors, etc.) or lubricating oils. O-rings fail due to a lack of lubrication and the inability of O-rings to adjust to extremely low temperatures.
The conditions under which gas pipe line companies operate today are different from those in existence twenty to forty-five years ago when many of the actuators currently in use were first put into service. Environmental regulations today call for as little discharge of natural gas into the atmosphere as possible. Fines for uncontrolled discharges can be very costly. All leaks must be held to a minimum. Compressor stations are becoming fully automated with only maintenance personnel on duty in the daytime five days a week. New government regulations are expected regarding main line break protection. In the past there has been automatic system protection or automatic systems that were used that reacted to a rate of change of pressure which closed the main line acutated valve. No provisions were available to re-open the main line unless operating personnel were dispatched to the main line valve. Current planning is to equip the pipe line system with more instrumentation so a central dispatcher knows everything occuring on the system and has the ability to close and re-open all critical valves. To do this, the existing gas motor powered valve actuators must be modified with remote control devices that are reliable and leak free.